Over the past decade in many countries, operating telephone companies have been investing extensively in a new generation of telephone switching equipment, generally referred to as digital switching systems. Such systems are characterized by an abundance of silicon based integrated circuits which act in concert to provide time division multiplex channels for coupling telephone sets in communication as required. Such systems are attractive because they require less space, less maintenance and attract greater revenue as compared to previous systems which provided for communications via metallic paths. Substantial design efforts have been committed to developing appropriate line interface circuits in the same technology. Recently transformerless analog silicon based integrated line interface circuits which are capable of performing coupling requirements between a switching system and a telephone set have become available. However many of the transformerless line circuit designs require a degree of precision and perfection in manufacturing, which typical state of the art integrated circuit manufacturers find difficult and expensive to meet. For example in U.S. Pat. No. 4,514,595, an "Active Impedance Line Feed Circuit", S.D. Rosenbaum et al disclose tip and ring amplifiers which are intended to feed tip and ring leads of a telephone line with the preferred direct current feed resistance of 400 ohms and the preferred AC impedance of 900 ohms. A control circuit drives the amplifiers in differential response to signals derived across matched tip and ring feed resistors which are coupled in series with outputs of the tip and ring amplifiers. The derived signals are coupled to a differential input of the control circuit via precision tip and ring voltage dividers. It is recommended that 0.05% tolerance feed resistors and 0.1% tolerance voltage divider resistors are preferred for interfacing telephone lines with a central switching office. Furthermore it is recommended that the differential input have the best possible common mode rejection characteristic so that longitudinally induced signals on the telephone line can be tolerated. Consequently the spectacular price and performance advantages realized in silicon based central office switching networks were not achieved to similar degree by this design of a line feed circuit.
In many switching systems, an individual line circuit is required to interface to each and every telephone line that is connected to the switching system. Hence, the cost of the line interface requirement has continued to be a major portion of the hardware cost of such switching systems. Furthermore in association with many telephone lines, the price/operating performance of many typical transformerless line interface circuits is often inferior to that of older designs as exemplified in U.S. Pat. No. 4,103,112 issued to V.V. Korsky on July 25, 1978. This is often due to the conditions in which a particular telephone line is required to operate. For example excessive longitudinal energy may be reactively coupled to the telephone line from nearby power utility distribution lines. Longitudinally induced currents on longer telephone lines may exceed the direct energizing current while the telephone set is in the OFF HOOK condition. This results in periodic reversals in the flow of the energizing current. In his U.S. Pat. No. 4,484,032, "Active Impedance Transformer Assisted Line Feed Circuit", S.D. Rosenbaum recognized some of the difficulties associated with his transformerless design and reintroduces a transformer. In this later development, his original design is modified in that feed resistor and voltage divider resistor recommended tolerances are loosened to 1% by using only the direct current component of the signals derived across the tip and ring feed resistors while differential AC signals are transformer coupled to the differential amplifier, substantially to the exclusion of longitudinal AC signals. However the tip and ring amplifiers are yet required to tolerate longitudinal currents on the telephone line. As the feed resistor may also be exposed to severe electrical stress, as can be the result of a random transient event such as a lightening strike or a power line cross, the values of the feed resistors may become altered beyond tolerance limits for acceptable line circuit operation.
It is an object of the invention to provide a line interface circuit wherein common mode signals are substantially impeded such that required common mode rejection characteristics of active components in the circuit are less critical.